Top Banner
The JET2000 project: Aircraft observations of the African easterly jet and African easterly waves. Thorncroft, C. D. 1 , Parker, D. J. 2 , Burton, R.R. 2 , Diop, M. 2 , Ayers, J. H. 3 , Barjat, H. 4 , Devereau, S., Diongue, A. 2 , Dumelow, R. 5 , Kindred, D.R. 4 , Price, N.M. 4 , Saloum, M. 6 , Taylor, C.M. 7 , Tompkins, A.M. 8 1 Department of Earth and Atmospheric Science, University at Albany, Albany, New York, USA 2 Institute for Atmospheric Science, School of the Environment, University of Leeds, Leeds, UK 3 HATS (MRF), DERA, Boscombe Down, UK 4 The Met Office, DERA, Farnborough, UK 5 The Met Office, Bracknell, UK 6 The Niger Met Service, Niamey, Niger 7 Centre for Ecology and Hydrology, Wallingford, UK 8 ECMWF, Reading, UK Submitted to Bulletin of the American Meteorological Society
58

JET2000: aircraft observations of the African Easterly Jet ... · Web viewThe aircraft observations were made along approximately 2.3oE approximately 4.5 hours after the observations

Feb 04, 2021

Download

Documents

dariahiddleston
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript

JET2000: aircraft observations of the African Easterly Jet system C D Thorncroft, D J Parker

The JET2000 project: Aircraft observations of the African easterly jet and African easterly waves.

Thorncroft, C. D.1, Parker, D. J.2, Burton, R.R.2, Diop, M.2, Ayers, J. H.3, Barjat, H.4, Devereau, S., Diongue, A.2, Dumelow, R.5, Kindred, D.R.4, Price, N.M.4, Saloum, M.6, Taylor, C.M.7, Tompkins, A.M.8

1 Department of Earth and Atmospheric Science, University at Albany, Albany, New York, USA

2 Institute for Atmospheric Science, School of the Environment, University of Leeds, Leeds, UK

3 HATS (MRF), DERA, Boscombe Down, UK

4 The Met Office, DERA, Farnborough, UK

5 The Met Office, Bracknell, UK

6 The Niger Met Service, Niamey, Niger

7 Centre for Ecology and Hydrology, Wallingford, UK

8 ECMWF, Reading, UK

Submitted to Bulletin of the American Meteorological Society

Corresponding author: Chris Thorncroft

e-mail: [email protected]

Abstract

Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 is presented. The Met Research Flight C130 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde and Niamey, Niger and two ‘box’ flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5-10o intervals. The two ‘box’ flights also included low-level flights that sampled north-south variations in boundary layer properties in the baroclinic zone beneath the AEJ.

Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross-sections of the African easterly jet (AEJ) on the 28th and the 29th (after the passage of a mesoscale system) are presented and discussed. Analysis of dropsonde data on the 28th, indicates contrasting convective characteristics north and south of the AEJ with dry convection more dominant to the north and moist convection more dominant to the south. The consequences of this for the AEJ and the relationship with the boundary layer observations are briefly discussed.

Preliminary NWP results indicate little sensitivity to the inclusion of the dropsonde data on the AEJ-winds in ECMWF and Met Office analyses. It is proposed that this may be due to a good surface analysis and a realistic model response to this. Both models poorly predict the AEJ in the 5-day forecast indicating the need for more process studies in the region.

The JET2000 aircraft campaign in West Africa is introduced along with preliminary analysis of the anomalous dry conditions that characterised the campaign, the African easterly jet, easterly waves and NWP data denial experiments.

Motivation

West Africa is a region that experiences marked interannual variability of rainfall (e.g. Rowell et al, 1995). This impacts water resources, agriculture and health, sometimes resulting in extreme social and economic problems and loss of life. West African climate variability also impacts the downstream tropical Atlantic by influencing hurricane activity (e.g. Landsea and Gray, 1992). Because of the marked variability and associated impacts there is a clear need for skilful medium to seasonal range predictions of rainfall and other variables for the West African region. There is also a need for increased confidence in predicted climate change scenarios for the region. The hydrological cycle in West Africa shows a large sensitivity to projected climate change scenarios and this at a time when water resources will come under intense pressure from a fast-increasing population (IPCC, 2001).

GCMs are the major tools used for weather and climate prediction. Current GCMs are hindered by large systematic errors in the West African region. For example, GCMs are known to poorly represent both the diurnal (e.g. Yang and Slingo, 2001) and annual cycle of rainfall over West Africa (e.g. see West African Monsoon Project final report (2001)). More process study work is required to increase our knowledge and understanding of the processes that determine the nature of West African weather and climate and its variability. Where possible this needs to link with research aimed at improving GCMs used for prediction. Focussed observing campaigns are required to shed light on key components of the West African monsoon that are not well observed by the routine network. One such component is the African easterly jet (AEJ), a mid-tropospheric jet present around 600mb and 15oN during the northern hemisphere summer.

The AEJ, present across the whole of West Africa (fig. 1(a)), has a key role to play regarding scale interactions in the West African and Atlantic regions. Its vertical shear is known to encourage the organised long-lived mesoscale convective systems (MCSs) (e.g. Houze and Betts, 1981) responsible for most of the daily rainfall events in the West African region. The AEJ-associated potential vorticity gradients and low-level temperature gradients satisfy the necessary instability criteria thought to give rise to African easterly waves (fig. 1(b,c) (e.g. Burpee, 1972), the major synoptic weather systems over West Africa that also trigger Atlantic tropical cyclones downstream. Studies on the interannual variability of West African rainfall have identified coherent signals in the variability of the AEJ (e.g. Newell and Kidson, 1984); whether the AEJ has an active or passive role in such rainfall variability is unknown. Despite the importance of the AEJ, it is poorly observed by the routine network (c.f. Fig. 12)

The JET2000 project capitalised on the fact that in the summer of 2000 the Met Office Research Flight (MRF) C-130 aircraft was stationed in Cape Verde, and was available to make observations over West Africa. Four flights with dropsondes, involving transects along and across the jet and the baroclinic zone were made. These observations are of unprecedented resolution for this part of the world. The aim of this paper is to describe the motivation and scientific background for the experiment and provide some preliminary findings.

Scientific background

Recent work on convection over the tropical oceans has suggested that boundary layer equivalent potential temperature (θe) and its variations has a strong influence on the nature of convection and circulations in the tropics (e.g. Emanuel et al, 1994). In this context, the Hadley cell arises in association with large-scale meridional gradients in boundary layer θe gradients, deep moist convection preferentially occurring in the region of higher θe.

Zheng et al (1999) applied these ideas to the West African monsoon. From their perspective, the West African monsoon arises in association with marked north-south boundary layer θe gradients that develop between the tropical land mass and the Gulf of Guinea and South Atlantic ocean. The situation is complicated by marked variations in surface properties over West Africa. North-south variations in albedo and vegetation between the desert to the north and rainforest to the south impact strongly on the surface fluxes and the boundary layer θ and θe distributions (c.f. Nicholson et al, 1998, Cook, 1999). As a result, peak values of boundary layer θ and θe, which are largely collocated over the ocean, are instead displaced over West Africa. θ peaks at around 25oN in the Sahara (c.f. fig1(c)) while θe peaks further south in the rainy zone around 10-15oN. Thorncroft and Blackburn (1999) discussed the consequences of this on convection. Deep moist convection characterises the high θe region while dry convection in the Saharan heat low characterises the high θ region. They argue that the mean tropospheric temperature profiles that are expected from an adjustment to a moist adiabat in the deep moist convecting region and a dry adiabat in the dry convecting region can help to explain the observed decrease in meridional temperature gradient with height, which in turn, through thermal wind balance explains the observed midtropospheric AEJ.

Thorncroft and Blackburn's model makes simple assumptions about the convection and the tropospheric temperature profiles. We need to know more about the nature of these profiles from observations and whether current GCMs can simulate or predict them. Observations and modelling studies need to be used to investigate the interaction between the dry and moist convecting regions. For example, it is likely that mid-level dry intrusions from the Sahara interact with the deep moist convecting zone affecting the nature of the convection there, analogous to the dry intrusions identified in the tropical West Pacific (e.g. Parsons et al 2000). Thorncroft and Blackburn's model also represents a synthesis of a balanced model and a turbulent convective model. Since the turbulence and convection are controlled by the diurnal cycle in the boundary layer, representation of the diurnal cycle seems to be crucial to the larger-scale evolution, yet is known to be dealt with badly in GCMs (e.g. Yang and Slingo, 2001).

If GCMs are to realistically predict the AEJ, they will need to realistically represent the interactions between the land surface, the boundary layer, the moist and dry convection and the dynamics. We do not know how well this is achieved in current GCMs used for weather and climate prediction because we have insufficient observational data to describe these interactions. Specifically, JET2000 was motivated by three science questions regarding the AEJ: (i) What is the state of balance between the AEJ, the turbulent convective boundary layer and the moist convecting atmosphere to the south? (ii) How does the diurnal cycle of the convective boundary layer influence the AEJ and monsoon flow? (iii) How do variations in the land surface properties impact on the convective boundary layer and the large scale dynamics?

While it is accepted that there is a strong, approximately zonally symmetric, forcing of the AEJ in association with meridional gradients in boundary layer θ and θe, it is well known that the AEJ is unstable to growing synoptic scale AEWs. As discussed in Smith et al (2001), very little research in recent years has explored the morphology and dynamics of such tropical weather systems that are of importance in daily to medium range weather forecasting (apart from tropical cyclones). We agree with Smith et al (2001) that forecasters in tropical regions have few conceptual models and that there is a notable lack of useful theory combining dynamics and moist convective processes. This is especially true in the West African region where the situation is hampered by lack of observational datasets. Operational forecasters are forced to combine the outputs from deficient NWP models with crude empirical rules. Our knowledge of the AEWs is dominated by three sources: the GATE data (e.g. Reed et al. 1977); NWP analyses (e.g. Reed et al 1988, Diedhiou et al, 1999); and theoretical models (e.g. Thorncroft and Hoskins 1994ab, Paradis et al, 1995). While Reed et al (1977) provided us with a valuable description of AEWs, our view is dominated by the composite analyses close to the West African coast. They provide a rather smooth picture, with little information about the evolution of individual waves. Operational analyses suffer from the sparsity of data going into the models and the model systematic errors. Theoretical models have helped us to develop conceptual models of AEWs but these must be developed in conjunction with new observations.

Research is needed to improve the NWP models on synoptic space and time scales, particularly in the representation of AEW structure and convection. Alongside this, we need to develop better conceptual models of AEWs for forecasting. Hence, in addition to questions about the AEJ, JET2000 was motivated by three questions about synoptic variability in the form of AEWs: (i) What is the dynamic and thermodynamic structure of AEWs and how do they evolve as they propagate along the AEJ? (ii) What is the relationship between the AEW structure, thermodynamic instability and convective rainfall? (iii) What is the impact of extra observations on NWP analyses and forecasts of the AEJ, AEWs and downstream tropical Atlantic?

The JET2000 questions are being addressed through a combination of analysis of aircraft observations and modelling efforts including data denial experiments. The MRF C-130 observations are particularly valuable since they include high resolution boundary layer measurements together with quasi-synoptic upper air observations over scales of several thousand kilometres. Although limited in temporal coverage to a period of less than one week, the data obtained using this platform have provided a unique resource with which to study the AEJ and AEWs. Some early results will now be described together with some logistics for the experiment.

Logistics

The Met Office MRF C-130 aircraft makes high frequency wind, thermal and humidity measurements, suitable for turbulence diagnostics, as well as measuring various bands of upward and downward radiation and observing certain aerosols and cloud condensation nuclei. In addition to the aircraft observations in JET2000, dropsondes were deployed for key sections of the flights, at 0.5 - 1. degree resolution; in total 112 dropsondes were deployed. Most were deployed successfully, although a minority of sondes yielded degraded wind data, especially in the first flight. During each flight the dropsonde data was transmitted to the data centre at the Met Office in Bracknell. This enabled the Met Office and ECMWF to assimilate the data in real time. The Met Office and ECMWF have subsequently made analyses and forecasts without this data to assess the sensitivity of including extra data in the region.

The experiment was planned in parallel with two other operations using the C-130; SHADE (Haywood et al, 2002) and SAFARI (Swap et al, 1998). The aircraft was available for JET2000 in the period 25-31 August 2000, during which approximately 35 hours of flying, on 4 flights, were available. This is the peak period for AEW activity in the rainy zone (e.g. Thorncroft and Hodges, 2001) and is close to the height of the Sahelian rainy season. Four flights were made; two west-east flights between Sal and Niamey and two principally north-south 'box' flights from Niamey (indicated in fig. 4). Flight 1 (Sal to Niamey) and flight 4 (Niamey to Sal) were intended to observe along-jet variability in association with AEWs. They were flown at maximum altitude possible for the aircraft (350-500mb), and deployed dropsondes at intervals of approximately one degree of longitude. Flights 2 and 3 (the box) were mainly aimed at making observations of the AEJ by deploying dropsondes with a spacing of 0.5 - 1 degree of latitude, together with aircraft measurements at low-levels. By flying twice over the same box, assessment of AEW structure and of local temporal variabilities was achieved. Low-level flights over the CATCH array were included. Flight 3 reversed the course to flight 2, with the low level flight preceding the high level box in order to estimate changes over the diurnal cycle.

The dates of each of the flights together with the timings for take off, landing and the times of the low-level flights are provided in table 1. The number of sondes released is also included for each flight. Flight 1 aimed to fly through an AEW trough before it reached the ocean. Flights 2 and 3 aimed to observe different phases of a wave while flight 4 aimed to fly through the same wave giving us a 4-dimensional view of the AEW. Consequently flights 2, 3 and 4 were conducted on consecutive days.

Operational flight planning was initiated in September 1999. Reconnaissance visits were undertaken to both of the planned operating airfields – Niamey in Niger and Sal in Cape Verde. During these visits, the planned flying was discussed with airport authorities and Air Traffic Control Units, who were all supportive of the project. There were to be a minimum of six and a possibility of eight countries involved with over-flights (including the release of dropsondes). The project was constrained in that diplomatic clearances could only be officially applied-for in the three weeks before departure. Given the amount of information and different requirements needed by each country, this resulted in an unusually large effort in exchanging informal information with the appropriate British Embassies in the months prior to the detachment. The local diplomatic representatives were able to 'sound out' their opposite numbers, in advance of the official diplomatic requests. This process was assisted by strong support from the local meteorological services. In the event, all of the diplomatic clearances were officially granted, the final ones in the week before the experiment!

The flight plans were reviewed on the day of the flights in light of the meteorological situation for that day. Daily analyses and forecasts were provided to the detachment by a dedicated forecaster at the Met Office in Bracknell, as well as through discussion with local forecasters. Up to date forecast information, particularly in regard to the evolution of convection, was relayed to the aircraft from Bracknell by SATCOM link.

Meteorological Situation During the Experiment

The JET2000 flights took place between 25th-30th August. Those of us who were looking forward to experiencing our first Sahelian squall line while in Niamey were disappointed. Between the time we arrived in Niamey in the afternoon on the 25th to the time we left in the morning on the 30th no rain fell on the city at all. In fact, before JET2000 the last significant rain recorded at Niamey airport was on the 17th August when 39.5mm fell. Although 2mm fell on the 30th (after we had left), it was not until the 20th September that more than 10mm fell again. Long term statistics for Niamey airport (Le Barbe and Lebel, 1997) indicate a mean August rainfall of 183mm and an average of 15 events (roughly one event every two days). We were clearly in Niamey during a dry period.

In order to give a slightly larger scale perspective of this dry period, we present in fig. 2 a time-series of the average rainfall based on the 34 rain gauges in the EPSAT network (a roughly 1ox1o square incorporating Niamey). The Niamey square was clearly characterised by more frequent and intense rainfall events before the start of the experiment. The last major storm of the season occurred on the 17th August, when an average of 29.5mm fell. It is also striking that for the period between the 24th August and 12th September only one day was characterised by a rainfall event of more than 5mm and most days had little or no rainfall at all confirming the extremely long dry period. It turns out that this dry period was felt over a wider region of the Sahel than just southern Niger (see figs. 3 and 4). As reported in the October food security bulletin of the USAID-funded Famine Early Warning System Network, the transition from wet to dry conditions around the middle of August lead to significant crop losses in the region emphasising the vulnerability of societies in West Africa to climate variability.

It is interesting to note that as well as the intraseasonal variations in rainfall, the Sahelian rainy season of 2000 was also characterised by intraseasonal variations in AEW activity. Although this can be seen in the radiosonde time-series at Niamey airport (not shown) it is most clearly illustrated in the time-longitude hovmoller of band-passed filtered meridional wind at 700mb based on ECMWF analyses (fig. 5). The Sahelian region experienced a sequence of coherent west-to-east moving AEWs during the last half of July to the middle of August, and then again in September. In between these times, roughly coinciding with the dry period, the AEWs were weaker and less coherent. This is consistent with the practical difficulties that were encountered diagnosing the AEW phase during the experiment. Whether this intraseasonal variability in AEW-activity is consistent with the rainfall variability is an area currently being investigated. Understanding this type of intraseasonal variability in rainfall and AEW activity is important for medium range weather prediction and may also be important for understanding the West African climate variability on seasonal and longer timescales.

We now present some preliminary analysis of the JET2000 period including some of the observations made during the experiment.

The African Easterly Jet

The first high-resolution observations of the African easterly jet structure were made on the second flight that took place on 28th August. Flight 2 consisted of a high level box between 8oN and 19oN (c.f. fig. 4 and fig. 12) with dropsondes released at approximately whole degree intervals on the western side and half degree intervals on the eastern side. This was followed by a low-level flight at around 875hPa between 9.8oN and 16.5oN on the eastern leg of the box. The AEJ observed on the eastern side, with a peak value of -21.3ms-1 is clearly evident at around 675hPa and 10oN (figure 6(a)). The AEJ is much stronger than the climatological average value of 15ms-1 often quoted and also located about 5o of latitude equatorward of the expected latitude for August (c.f. Reed et al (1977)). Newell and Kidson (1984) and more recently Grist and Nicholson (2001) suggest that the AEJ is stronger and equatorward of its mean position during dry Sahelian years. Whether the strong and equatorward AEJ observed on flight 2 is consistent with the dry period in the region remains to be determined.

Figure 6(a) also shows marked baroclinicity above and below the AEJ consistent with the marked vertical shear there. The baroclinic zone slopes downwards towards the Sahara and the strongest shear is consistent with this. Above the sloping baroclinic zone, in contrast, there is very little vertical shear in a region of low static stability, consistent with mixing of momentum during dry convection. The deep well-mixed layer above the surface at around 19oN is characterised by a potential temperature of about 315K. Whereas at this latitude this well-mixed layer developed locally, a well-mixed ‘wedge’ with similar potential temperature values exists above the sloping baroclinic zone suggestive of equatorward movement of air from the active dry convecting region towards the moist convecting region. This is consistent with fig. 6(b) which shows the humidity mixing ratio for the same section. Extremely dry air characterises the low stability region around 19oN as expected; but the section also indicates a sloping transition zone between this dry air and the moister air equatorwards and beneath. A more pronounced dry slot which may be the result of dry advection from the Saharan region can also be seen sloping upwards from about 800hPa at about 14oN. More research is required, including trajectory analyses, in order to identify more precisely the origin of such dry slots; whether they originate from the Sahara or midlatitudes or whether they arise from the actions of downdrafts during moist convection. It is important to identify whether the dry air seen in fig. 6(b) around 14oN originated from outside the region since it may have had some role to play in the persistence of the dry period in the Niger region.

Aircraft observations of the low-level θ and θe along this same section made a few hours later are included in fig. 7 (as solid lines). The low-level θ increases polewards as expected, with a significant increase in the gradient polewards of about 12oN. The low-level θe peaks around 12.5oN and generally decreases polewards of this. As discussed above, these gradients are linked to the north-south variations in the land surface, from a well-vegetated surface with high soil moisture content in the south, to the Sahara desert with sparse vegetation and low soil moisture in the north. Associated with this, we expect north-south variations in convection and associated vertical thermodynamic profiles. This is illustrated in fig. 8, which shows tephigrams based on dropsondes at 19oN and 8oN, latitudes we expect to be characterised by dry convection and moist convection respectively. Consistent with this we can see rather strikingly that the temperature profile at 19oN is dominated by a deep dry adiabatic layer between 625mb and 850mb with a new layer developing beneath. In contrast, the profile at 8oN is characterised by a profile closer to pseudoadiabatic with higher humidities as expected. The result is that the two temperature profiles cross at around 625hPa close to the observed AEJ height in fig. 6(a) confirming, in a qualitative sense, the conceptual model of Thorncroft and Blackburn (1999). However, the profile at 8oN is warmer and drier than the saturated moist adiabat with the local boundary layer value of θe proposed in this conceptual model. The extent to which this typifies the region during the season and the physics that determine these profiles including the role of dry intrusions will be examined using GCM analyses, available observations and idealised simulations.

An important goal of the JET2000 project is to assess whether GCMs, used for weather and climate prediction, can adequately reproduce the observed low-level gradients in θ and θe such as those seen in fig. 7 and the atmosphere’s response to them such as that illustrated in figs 6(a) and 8.

The Saharan Air Layer

A major feature of the West African monsoon, already illustrated by the dropsonde at 19oN on the 28th August (fig. 8), is the deep dry adiabatic layer that develops over the Sahara. This layer has often been referred to as the Saharan air layer (SAL) (e.g. Karyumpudi and Carlson, 1988) and has motivated research both due to its impact on the West African monsoon but also its impact on the Atlantic weather and climate (e.g. Prospero and Nees, 1986). Indeed, the SAL was very clearly seen on the first flight out of Sal on the 25th August. Leaving Sal, the aircraft profiled from 50ft above the ocean to approximately 500hPa. Figure 9 shows the sounding for this aircraft ascent. It shows a very deep mostly dry adiabatic layer above a cooler moist boundary layer consistent with an overrunning of the oceanic boundary layer by the SAL. The deep adiabatic layer was characterised by significant haze in association with Saharan dust observed up to the inversion at about 520mb. It has been suggested that the dryness and high aerosol content of the layer may be important for inhibiting moist convection and tropical cyclogenesis in the tropical Atlantic (Prospero, personal communication).

It is also important to note that the layer may also have a dynamical impact. As discussed by Thorncroft and Blackburn (1999) the low-static stability in the layer results in a very low value of potential vorticity (PV). This can be seen in the August 2000 mean (fig. 1(b)) where low PV characterises the region polewards of about 15oN over the continent but also over the tropical East Atlantic. The low PV represents a significant negative PV anomaly in the region and consistent with this, is associated with anticyclonic relative vorticity on the poleward side of the AEJ. Indeed, the low PV of the SAL is a key part of the PV-sign-reversal that characterises the instability of the AEJ (e.g. Thorncroft and Blackburn, 1999, Dickenson and Molinari, 2000).

The African Easterly Waves

Despite the weak AEWs during JET2000 (c.f. fig. 5) coherent wavelike behaviour could still be diagnosed, especially towards the end of the experiment. To illustrate these waves fig. 10(a) shows the unfiltered meridional wind based on the ECMWF analysis at 700hPa on the 30th August. A sequence of AEWs is evident in this analysis with well-defined troughs around 30oW and 12oW and a broadscale trough around 5-10oE.

The trough over the ocean is well-defined and was associated with the development of hurricane Ernesto a few days later. The weak trough close to the West coast was observed during JET2000. Flights 2 and 3 flew meridional transects in and behind it during its passage past Niamey and flight 4 flew through it on the way back to Sal. The trough east of this has a more complicated structure with wind maxima at various latitudes between the equator and 25oN. While one must be a little cautious of the analysis due to the data sparsity of this region, the multi-centred nature of AEWs is expected and is associated with AEJ-level PV anomalies and low-level temperature anomalies (c.f. Pytharoulis and Thorncroft, 1999). The structure may be further complicated through the impact of mesoscale convective systems (e.g. Houze and Betts, 1981) , orography (e.g. Mozer and Zehnder, 1996) or midlatitude trough intrusions.

Given the relatively coherent analysis of the AEWs on the 30th it is natural to wonder whether these waves were associated with a coherent pattern of convection. The infrared satellite image at this time (fig. 10(b)) indicates marked variations in convective activity from west to east. The troughs diagnosed around 30oW and 12oW appear to have a marked convective signature. There is also a broad region of convection east of this around 15oE but it is uncertain whether this is linked to the passage of the broadscale trough. More detailed analysis of the temporal evolution of AEWs is needed in order to assess whether, in general, AEWs have a coherent and predictable relationship with convection and to determine the key processes that influence this relationship.

A Mesoscale Convective System

Planning for the third flight on 29th August was dominated by discussion about an MCS approaching the southern sector of the box from the east in the early morning. After discussions with the forecasters at Niamey airport, it was estimated that the MCS would intersect the box around 1100-1200 UTC. In order not to jeopardise the high level box, it was decided to shorten the low level flight to a transect from 14.1oN to 9.8oN, so that the aircraft could fly north on the high level box while the MCS was in the vicinity of the box to the south. Then the aircraft would close the box when the MCS had moved to the west. This strategy also had the attraction that we may be able to make observations both ahead of and behind the MCS: an important consideration in assessing the influence of the convection on the AEJ. In the event, the planning was entirely successful: the MCS decayed over Benin, but regenerated in the vicinity of the Atakora hills (just to the west of the western leg, at around 11oN)) and observations were made in advance of and behind the storm.

Figure 11 shows the north-south vertical section of zonal wind and θ for the eastern side of the box which was made after the passage of the MCS. There are significant differences between this section and the same one on the previous day. The AEJ on this day is still equatorward of what we may have expected at this time of year but it has a different structure from that observed on flight 2. It has a split structure with peak values of 19.9ms-1 and 21.0ms-1 around 650hPa and 9.5oN and 12.5oN respectively. Between these latitudes around 11oN there are marked easterlies around 825hPa and below which are consistent with the action of a mesoscale downdraft Associated warming, as depicted by the bowing down of isentropes, and drying (not shown) is further evidence for the downdraft to the rear of the MCS. Figure 11 serves as a reminder of the two-way interactions that can take place between the AEJ and MCSs. While the vertical shear associated with the AEJ is important for encouraging organised MCSs, the MCSs themselves can also impact the AEJ. Whether GCMs can adequately resolve or parametrise such interactions or whether they need to are open questions that need investigating with parallel mesoscale and GCM modelling studies.

Preliminary NWP Results

West Africa, along with most of the continent, is a well-known data sparse region with very few routinely launched radiosondes. Figure 12 shows the geographical location of all the sondes that were assimilated into the ECMWF model at 12UTC on the 28th August. The 25 dropsondes that were assimilated from flight 2 show up rather dramatically. While the routinely launched radiosondes may be useful for analysing along-jet variations (west of 10oE) at around 14oN they are clearly inadequate for resolving the significant north-south variations that exist in the region of the AEJ. The significance of this for NWP over Africa and downstream needs to be investigated: we aim to assess the impact of the extra dropsonde observations made during JET2000 on analyses and forecasts made using the operational ECMWF and Met Office models.

Figure 13 shows the ECMWF model analyses with and without the dropsondes assimilated. What is immediately striking is that the ECMWF analysis without the dropsondes is actually quite good, the AEJ maximum being within 24% of the strength observed and its position being located to within 20 hPa and 0.9 degrees latitude (c.f. Fig 6(a) and table 2). This is despite the fact that there are no radiosondes in the vicinity of the observed AEJ. Including the dropsondes in the ECMWF data assimilation strengthens the AEJ by 1.6ms-1, shifts it to a new position now slightly equatorward of the observed AEJ and creates a smoother-looking AEJ. The estimates of vertical shear (in table 2) show quite good agreement between both analyses and observations but this is due to compensating errors with a weaker AEJ located slightly lower down than observed and low-level easterlies that are too weak. The Met Office analyses (summarised in table 2) also shows very good agreement with the dropsonde observations. Including the dropsondes in the analysis does not significantly improve the analysis of the AEJ-winds because it was so close to the observations already (although other aspects of the analysis may have improved, as in the ECMWF analysis of low-level moisture indicated in fig.7 for example).

It would be wrong to speculate too much regarding the differences seen here between the ECMWF and Met Office analyses before more work is done. However it is clear that differences in model sensitivities to the dropsonde data may arise from many sources. These include model differences, assimilation differences or differences in the total amount of data used (e.g. inclusion of other data such as satellite observations) and random fluctuations. Differences between the 3D and 4D variational assimilation schemes used by the Met Office and ECMWF respectively were noted in the analysis of FASTEX cases (Desroziers et al, 1999) and could also be contributing to the differences seen here. The high quality of the analyses without the dropsondes, despite the lack of routinely reporting radiosondes (see fig. 12), may be fortuitous or may be due to a good analysis of surface conditions and a realistic model response to them. This is currently being investigated through more data denial experiments.

Forecasts for 12UTC 28th August

The sparse routine observational network in West Africa means that it is difficult to evaluate how well GCMs predict the AEJ and associated AEWs. JET2000 observations give us an opportunity to do this for the period of the experiment. We choose to illustrate here a problem that both the ECMWF and Met Office models have with predicting the AEJ several days in advance. The 5-day forecasts of the AEJ in the JET2000 region for 12 UTC 28th August are shown in fig. 14. In these forecasts, it is very difficult to see an AEJ at all. Instead we see a region of weak broadscale easterlies with erroneous mid-level easterlies north of 15oN. It is intriguing that both the ECMWF forecast and the Met Office 5-day forecast have very similar problems with a very weak AEJ and associated weak vertical and horizontal shears. Assuming that the starting analysis for the forecast (valid at 12UTC 23rd August) was as good as the analysis for 12UTC on 28th August, then it is a concern that in the space of 5 days, both models can move so far from the observed and climatological states. This clearly indicates that some processes are being misrepresented in the region. Indeed, analysis of the ECMWF 5-day forecast indicates that the low-level meridional temperature gradient is too weak in association with an erroneously cool and moist boundary layer polewards of about 13oN (see fig. 7). This error is consistent with the systematic 5-day forecast errors for August 2000 (not shown) emphasising the value and need for process studies in this region.

In summary, both the ECMWF and Met Office models were able to represent an AEJ which was considerably removed from its climatological position, despite the absence of upper air observations at this latitude. The study has also indicated however, that the model forecasts appear to exhibit significant drift away from model analyses and climatology. Ongoing work associated with JET2000 aims to investigate these findings by identifying those data streams which are enabling a good analysis and by identifying the processes which explain the drift in the model forecasts.

Final Comments and Future Plans

The observations from the JET2000 experiment are a valuable scientific resource for study of this data sparse region of the world and so all are available to scientific community without delay (see BADC website at http://www.badc.rl.ac.uk/data/jet2000). We still recognise the need for more observational studies in this data sparse region. It is hoped that JET2000 research and experiences gained will help in planning such experiments. Indeed, a major international research and field campaign is currently being planned for the West African region and will span several years starting in 2004. This project has become known as AMMA (African Monsoon Multidisciplinary Analysis) and is concerned with weather and climate variability in the region and how this impacts such things as water resources, food security, health, chemistry and tropical cyclones. More details of the AMMA project can be found at : http://medias.obs-mip.fr:8000/amma/english/index_en.html

Acknowledgements

This experiment was funded by NERC under grant GR3/13118. C. Thorncroft has been partially funded for this work by NSF (PTAEO: 1023911-1-24796).

We would like to acknowledge the support we received at the Cape Verde and Niger Meteorological Services. In particular we would like to personally thank Mr Soares from Cape Verde. We would also like to thank the support and hospitality we received from ACMAD in Niger, in particular Mr Boulaya and Mr Afiesimama.

We would also like to acknowledge the input of the aircrew and aircraft scientists of the Meteorological Research Flight: Martin Cook, Pat Coyle, Rob Gregory, Stuart Heath, Mike Kempster, Julian Pantrey, Dave Pearce, Derek Percival, Martyn Pickering, Ken Quick, Tony Simpson, Phil Summers, Ian Woodford and Paul Woodman. We would like to acknowledge Jason Lander for computing support, Iain Russell and TAMSAT for providing us with Meteosat imagery used during the experiment and in fig. 10 and Richard Forbes from the Met Office for supplying the code for the analysis of the dropsondes. The OLR data used in figs. 3 and 4 was obtained from UCAR.

References

Brovkin, V., Claussen, M., Petoukhov, V., and Ganopolski, A. 1998. On the stability of the atmosphere-vegetation system in the Sahara/Sahel region. Journal Of Geophysical Research-Atmospheres 103(D24), 31613-31624 .

Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci., 29, 77-90.

Cook, K.H. 1999 Generation of the African easterly jet and its role in determining West African precipitation. J. Climate, 12, 1165-1184.

Desroziers, G., Pouponneau, B., Thepaut, J.-N., Janiskova, M. and Veerse, F. 1999 Four-dimensional variational analyses of FASTEX situations using special observations, Q.J.R.Meterol.Soc., 125, 3339-3358.

Dickenson, M. and Molinari, J. 2000 Climatology of sign reversals of the meridional potential vorticity gradient over Africa and Australia. Mon. Wea. Rev., 128, 3890-3900

Diehdiou, A., Janicot, S., Viltard, A., de Felice, P. and Laurent, H. 1999 Easterly wave regimes and associated convection over West Africa and the tropical Atlantic: Results from NCEP/NCAR and ECMWF reanalyses. Climate Dynamics, 15, 795-822.

Dirmeyer PA and Shukla J. 1996. The effect on regional and global climate of expansion of the worlds deserts. Quart.J.Roy.Meteorol.Soc. 122: 451-482.

Emanuel, K.A., Neelin, J.D. and Bretherton, C.S., 1994 On large-scale circulations in convecting atmospheres, Q. J. R. Meteorol. Soc., 120, 1111-1143.

Goldenberg, S.B. and Shapiro, L.J. 1996 Physical mechanisms for the association of El Nino and West African rainfall with Atlantic major hurricane activity. J. Climate, 9, 1169-1187.

Grist, J. P. and Nicholson, S. E. 2001 A study of the dynamic factors influencing rainfall variability in the West African Sahel, J. Clim., 14, 1337-1359

Haywood, J., Francis, P., Osborne, S., Glew, M., Loeb, N., Highwood, E., Tanre, D. and Formenti, P. 2002 Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1 Solar Spectrum. Submitted to JGR

Houze and Betts, 1981 Convection in GATE. Rev. Geophys. Space Phys., 19, 541-576.

IPCC 2001: Climate Change 2001: Impacts, adaptation and vulnerability – contribution of working group II to the third assessment report of the Intergovernmental Panel on Climate Change (IPCC), WMO, pp1000 (available at http://www.grida.no/climate/ipcc_tar/wg2/index.htm).

Karyampudi, V.M. and Carlson, T.B. 1988 Analysis and numerical simulations of the Saharan air layer and its effect on easterly wave disturbances. J. Atmos. Sci., 45, 3102-3136

Landsea, C.W. and Gray, W.M. 1992 The strong association between Western Sahel monsoon rainfall and intense Atlantic hurricanes. J Clim., 5, 435-453.

LeBarbe, L. and Lebel, T. 1997 Rainfall Climatology of the HAPEX-Sahel region during the years 1950-1990. J. Hydrol., 189, 43-73

Lebel, T. and LeBarbé L., 1997. Rainfall monitoring during HAPEX-Sahel :2. Point and areal estimation at the event and seasonal scales. Journal of Hydrology. 188-189, 97-122.

Mozer, J.B. and Zender, J.A. 1996 Lee vorticity production by large-scale tropical mountain ranges Part II: A Mechanism for the production of African Waves. J. Atmos. Sci., 53, 539-549

Newell, R.E. and Kidson, J.W. 1984 African mean wind changes between Sahelina dry and dry periods. J. Climatol., 4, 27-33.

Nicholsen, S.E., Tucker, C.J. and Ba, M.B. 1998 Desertification, drought, and surface vegetation: an example from the West African Sahel. Bull. Amer. Met. Soc., 79, 815-829

Parsons,D.P., Yoneyama,K. and Redelsperger,J-L. 2000 The evolution of the tropical western Pacific atmosphere-ocean system following the arrival of a dry intrusion. Q.J.R.Meteorol.Soc., 126, 517-548.

Pedder, M. 1993 Interpolation and filtering of spatial observations using successive corrections and Guassian filters. Mon. Wea. Rev., 121, 2289-2902.

Prospero, J. M. and Nees, R. T. 1986 Impact of North African drought and El Nino on mineral dust in the Barbados trade winds, Nature, 320, 7345-738

Pytharoulis, I. And Thorncroft, C.D. 1999 The low-level structure of African easterly waves in 1995. Mon. Wea. Rev., 127, 2266-2280

Reed, R.J., Norquist, D.C. and Recker, E.E. 1977 The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev., 105, 317-333.

Reed,R.J., Klinker,E. and Hollingsworth, A. 1988 The structure and characteristics of African easterly wave disturbances determined from ECMWF operational analysis/forecast system. Meteorol.Atmos.Phys.,38,22-33

Ringer, M.A., 1998 Tropical convective rainfall in the global UM: Initial comparisons with estimates derived from Meteosat infrared imagery, UK Meteorological Office, Forecasting Research Technical Report No. 239.

Rowell, D.P., Folland,C.K., Maskell, K. and Ward, M.N. 1995 Variability of summer rainfall over tropical north Africa (1906-92): Observations and modelling. Q.J.R.Meteorol.Soc., 121, 669-704.

Smith, R.K., Garden, G., Molinari, J. and Morton, B. (2001) Proceedings on an International Workshop on the Dynamics and Forecasting of Tropical Weather Systems. Bull. Amer. Met. Soc., 82, 2825-2829

Sperber,K.R. and Palmer,T.N. 1996 Interannual tropical rainfall variability in GCM simulations associated with AMIP. J.Clim., 9,2727-2749

Swap, B., J. Privette, M. King, D. Starr, T. Suttles, H. Annegarn, M.Scholes and C.O. Justice (1998), SAFARI 2000: a southern African regional science initiative,EOS Earth Observer, 10(6):25-28.

Taylor C.M. and Lebel T. 1998. Observational evidence of persistent convective-scale rainfall patterns. Mon.Weath.Rev. 126: 1597-1607.

Thorncroft, C.D. and Blackburn, M., 1999 Maintenance of the African easterly jet, Q. J. R. Meteorol. Soc., 125, 763-786.

Thorncroft, C.D. and Hodges, K.I. 2001 African easterly wave variability and its relationship to Atlantic tropical cyclone activity, J. Climate,14, 1166-1179

Thorncroft, C.D. and Hoskins, B.J., 1994a An idealised study of African easterly waves Part 1: A linear view, Q. J. R. Meteorol. Soc., 120, 953-982.

Thorncroft, C.D. and Hoskins, B.J. 1994b An idealised study of African easterly waves Part 2: A non-linear view, Q. J. R. Meteorol. Soc., 120, 983-982.

Yang, G. and Slingo, J. 2001 The diurnal cycle in the tropics. Mon. Wea. Rev., 129, 784-801.

Zheng, X.,Eltahir, A.B. and Emanuel, K.A. 1999 A mechanism relating tropical Atlantic spring sea surface temperature and west African rainfall. Q.J.R.Meteorol.Soc., 125, 1129-1163.

Zipser, E.J. 1977 Mesoscale and convective downdraughts as distinct components of squall line structure. Mon. Wea. Rev., 1568-1589.

Figure Captions

Figure 1. Mean fields for August 2000 based on operational ECMWF analyses: (a) Zonal wind at 700hPa with a contour interval of 2ms-1 and values greater than 6ms-1 shaded, (b) Ertel potential vorticity at 315K (close to 700hPa at 15oN) with a contour interval of 0.05PVU and values greater than 0.2PVU shaded and (c) Potential temperature at 925hPa with a contour interval of 2K and values greater than 312K shaded. The positive and negative meridional PV gradients associated with the PV maximum on the equatorward side of the AEJ, combined with the positive meridional gradients in low-level potential temperature satisfy the necessary condition for mixed barotropic-baroclinic instability. See Thorncroft and Hoskins (1994a) for more discussion of this and the significance for AEW growth.

Figure 2 Time-series of the mean rainfall based on 34 raingauges in the EPSAT square. The data is averaged over 24 hours and the stations are located between 1.7oE and 3.1oE and 13.0oN and 13.9oN.

Figure 3. Time-latitude hovmoller showing outgoing long-wave radiation (OLR) averaged between 10oW and 10oE. Shading indicates OLR values less than 220Wm-2 with darkest shading indicating values less than 140Wm-2. Low values of OLR are consistent with high clouds which we use here as a proxy for rainfall. The figure illustrates the weak convective activity over the Sahelian region between the middle of August and the beginning of September.

Figure 4. A map of West Africa showing the difference between the mean OLR (in Wm-2) for the period between 20th to 31st August and a climatology of the same period (based on 1990 to 1999). Positive differences over much of the region including the central Sahel and Guinea Coast are indicative of relatively drier conditions during the latter half of August and illustrate the large area that experienced a dry period. Wetter conditions are indicated in the West Sahel. Also included in the figure is a schematic of the four flights made during the JET2000 experiment. Flights 1 and 4 were between Sal and Niamey. Flights 2 and 3 were ‘box-flights’ starting and finishing in Niamey.

Figure 5: Time-longitude hovmoller showing band-passed (2-6 days) filtered meridional wind at 700hPa based on 00UTC and 12UTC ECMWF operational analyses (includes dropsondes). For clarity only southerlies greater than 2ms-1 are shaded. Dark shading indicates values greater than 4ms-1. The figure illustrates the weaker AEW-activity over West Africa between the middle of August and the beginning of September.

Figure 6. Pressure-latitude sections of (a) zonal wind (shaded) and potential temperature with a contour interval of 2K and (b) humidity mixing ratio based on dropsonde data for flight 2 on 28th August 2000. The section shown is for the eastern leg of the box shown in fig. 2 along approximately 2.3oE. Location of dropsonde release locations are indicated as red triangles. The objective analysis used follows the successive correction method described by Pedder (1993). The blackened regions along the top and bottom of the figure denote the extent of the data coverage.

Figure 7. Meridional profiles of (a) potential temperature,θ and (b) equivalent potential temperature, θe based on aircraft observations on the 28th August at a height of approximately 875hPa (solid), between 1317-1538UTC, ECMWF analysis for 12UTC 28th August with the dropsondes (long dashed), ECMWF analysis for 12UTC 28th August without dropsondes (small dashed) and the 5-day forecast from ECMWF for 12UTC 28th August (dotted). The aircraft observations were made along approximately 2.3oE approximately 4.5 hours after the observations presented in fig. 6 were made (c.f. table 1), during which time the boundary layer was warmed by a few degrees (consistent with offset). Data shown is averaged over 5 minutes. ECMWF data is based on a model level which varies between 890hPa (at 9N) and 860hPa (at around 17N) and is averaged between 1.5E and 3.5E.

The observed θ profile is characterised by a weak meridional gradient up to about 12oN and a stronger positive gradient polewards of this. The meridional θ gradient is generally well captured by the analyses, perhaps consistent with the good analysis of the AEJ. The meridional gradient in the 5-day forecast is weaker in association with a cold error increasing with latitude polewards of about 11oN.

The observed θe profile is characterised by a peak around 12.5oN and a marked reduction polewards of this consistent with increasingly drier conditions. Both analyses indicate significant departures from observations polewards of about 14oN, with anomalously high values and peaks around 16oN. The analysis without the dropsondes is particularly poor with its anomalous peak even higher than the observed equatorward peak. These errors are associated with erroneously moist conditions at these latitudes.

The meridional profiles of θ and θe in the 5-day forecast exaggerate the errors seen in the analysis with large cold and moist errors, especially polewards of about 13oN. The resulting weaker θ gradient is consistent with the weaker predicted AEJ (c.f. fig. 14).

Figure 8: Tephigrams showing temperature (solid) and dew point temperature (dashed) based on dropsondes 19oN (bold) and 8oN (normal) during the eastern leg of flight 2 on 28th August.

Figure 9. Tephigram showing temperature (solid), dewpoint temperature (dashed) and winds for the aircraft ascending profile from Sal on 25th August 2000.

Figure 10. (a) Operational ECMWF analysis at 12UTC 30th August 2000 of unfiltered 700hPa meridional wind with a contour interval of 1ms-1 , (southerlies are shaded with dark shading for winds greater than 4ms-1) and (b) the infrared satellite image at the same time from METEOSAT.

Figure 11. Pressure-latitude section of zonal wind and potential temperature based on dropsonde data for flight 3 on 29th August 2000. Zonal wind is shaded and potential temperature is contoured with an interval of 2K. The section shown is for the eastern leg of the box shown in fig. 4 along approximately 2.3oE. Location of dropsonde release locations are indicated as red triangles. . The objective analysis used follows the successive correction method described by Pedder (1993). The blackened regions along the top and bottom of the figure denote the extent of the data coverage.

Figure 12. Distribution of radiosondes (circles) and JET2000 dropsondes (triangles) in the north African region that were assimilated into the ECMWF Model in the 12UTC analysis on 28th August 2000.

Figure 13. Pressure-latitude sections of zonal wind averaged between 1.3E and 3.3E for 12UTC 28th August 2000: (a) ECMWF analysis with assimilated dropsondes, and (b) ECMWF analysis without assimilated dropsondes. The model assimilation was performed at T511 resolution, which is equivalent to a 0.3o grid and had 60 levels in the vertical. The plot shown here uses a 0.5o grid in the horizontal but includes the 60 levels.

Figure 14. Pressure-latitude sections of zonal wind averaged between 1.3E and 3.3E for 12UTC 28th August 2000 and based on T+120hour forecasts from (a) ECMWF and (b) The Met Office.

Table 1: Flight summary table, all times GMT.

Flight Code

Date

Take Off

Landing

Low-level

No. of sondes

1

25/8/00

09:09

15:25

None

21

2

28/8/00

07:01

16:20

13:17-15:38

35

3

29/8/00

06:58

15:03

07:14-08:43

34

4

30/8/00

09:55

15:45

None

22

Table 2: AEJ characteristics from observations and analyses. Vertical shear is based on the zonal wind difference between the peak easterly and the zonal wind at 925hPa.

AEJ

VARIABLE

DROPSONDES

RAW DATA

MET OFFICE

WITH

MET OFFICE

WITHOUT

ECMWF

WITH

ECMWF

WITHOUT

Maximum

-21.3ms-1

-19.1ms-1

-20.2ms-1

-17.8ms-1

-16.2ms-1

Height

650hPa

630hPa

630hPa

670hPa

670hPa

Latitude

10.0N

9.6N

9.8N

9.5N

10.9N

Vertical shear

Below AEJ

6.2 ms-1 /100hPa

5.3ms-1 /100hPa

6.0ms-1 /100hPa

6.4ms-1 /100hPa

6.4ms-1

/100hPa

� http://www.atmos.albany.edu/facstaff/chris/wampreport_WORD.html

� � HYPERLINK http://www.lthe.hmg.inpg.fr/catch ��http://www.lthe.hmg.inpg.fr/catch�

� http://www.fews.net

� This peak value is taken from the raw data. The peaks in fig. 6(a) are weaker due to smoothing by the successive correction analysis. See figure caption for more details.

PAGE

24